Polyimides which exhibit excellent thermooxidative properties and which therefore are contemplated for use in making carbon fiber composites employed in the aerospace industry are currently available and have been known for some time as illustrated by U.S. Pat. Nos. 3,528,950; 3,575,924; 3,671,490; and 3,745,149. At least some of the polyimides described in these patents are similar to the extent that they are derived from three basic types of monomers, such as, a dianhydride or ester derivative thereof, a polyfunctional amine, and an unsaturated monoanhydride or ester derivative thereof.
The processes for preparing these polyimides, however, vary considerably. For example, U.S. Pat. Nos. 3,575,924 and 3,671,490 disclose a process wherein an intermediate polyamide acid is formed by the reaction of a dianhydride and a diamine in an anhydrous solvent. The polyamide acids thus produced are then converted into polyimides by reaction with an unsaturated anhydride which occurs in one or two reaction stages. In one embodiment, polyamide acid and unsaturated anhydride are heated to a temperature between 100.degree. and 450.degree. C. in a single operation. In another embodiment, the polyamide acid and unsaturated anhydride are subjected to a cyclizing dehydration using a dehydrating agent, to form a linear polyimide which is then heated to a temperature between 80.degree. and 350.degree. C. A peroxide catalyst such as benzoyl peroxide may be employed in either embodiment for carrying out the reaction of the polyamide acid but not during the formation of the polyamide acid.
The polyamide acid which is formed as described is unstable and must be kept in solution, hermetically sealed and refrigerated if not utilized within a few hours. Special handling and storage greatly increases the ultimate cost to the user. Such polyamide acids require curing by heat treatment to the final polyimide for periods of from 5 to 16 hours and such curing results in the evolution of appreciable amounts of volatile materials.
U.S. Pat. No. 3,528,950 discloses a method for preparing high molecular weight polyimides without the necessity of forming the polyamide acids. Thus, a low molecular weight prepolymer is prepared by reacting a polyfunctional amine, a poly-functional anhydride, and an unsaturated monoanhydride, such as nadic anhydride, by refluxing for a period of 18 hours.
Such treatment yields two polyimide prepolymers, one of a higher molecular weight and a second of a lower molecular weight, which are subsequently blended in dry-powder form. The blend of prepolymer is then heated to a temperature of 200.degree. to 350.degree. C. to form polyimide macro-molecules. While utilization of such prepolymers eliminates the instability problems that are encountered with the polyamide acid approach, the preparation of the prepolymer is very time-consuming and requires the separate step of blending the two prepolymers that are formed in the refluxing step prior to heating to cure the macro-molecular polyimide.
U.S. Pat. No. 3,745,149 discloses a process for preparing certain polyimides by heating a mixture of solvent, and monomer compounds which include an ester derivative of a tetracarboxylic acid, a diamine, and an ester derivative of an unsaturated monoanhydride, such as nadic anhydride. The monomer-solvent mixture may be heated at temperatures of 50.degree. to 205.degree. C. (i.e., 122.degree. to 401.degree. F.) to form a low molecular weight prepolymer. The prepolymer can then be heated at temperatures of about 275.degree. to about 350.degree. C. (i.e., 527.degree. to 662.degree. F.) to obtain chain extension and/or cross-linking of the prepolymer. Alternatively, the monomer mixture may be heated initially at temperatures of about 275.degree. to 350.degree. C. for a period of about 30 minutes to one hour to obtain the crosslinked high molecular weight polyimide. The method described in this patent avoids the need to form the unstable polyamide acid. However, very high temperatures are required to achieve final cure of the polymer over extended periods of time.
For example, a particularly preferred polyimide can be prepared from a mixture of monomers known as LARC-160.
These monomers include benzophenone tetracarboxylic acid diester (BTDE), Jeffamine AP-22.sup.TM, and the monoethylester of nadic anhydride. These three monomers have heretofore required an extensive and time-consuming cure cycle wherein the monomers are slowly heated to temperatures of about 285.degree. to about 350.degree. F., e.g., 325.degree. F., and maintained at this temperature for one hour to form the prepolymer. The temperature is then slowly raised to 600.degree. F. (301.degree. C.) and maintained thereat for an additional two hours. At the end of the cure cycle, the material is subjected to a four hour post cure at 600.degree. F. (301.degree. C.).
Other patents which disclose high temperature resistant polyimides include U.S. Pat. Nos. 3,772,254 and 4,110,294.
It would be a distinct advantage if it were possible to lower the temperature of the second stage of the cure cycle of polyimides, such as those derived from LARC-160 (discussed in detail hereafter), below that currently employed to temperatures of about 350.degree. to 400.degree. F. This would permit the use of the same bagging and adhesive materials currently used in the autoclave curing of epoxies. At elevated cure temperatures much more expensive sealers and vacuum bags would have to be employed. The ability to employ low temperature epoxy curing techniques and apparatus would make it possible to take advantage of the potentially better stability of polyimides when exposed to use temperatures of about 160.degree. to about 180.degree. F. in humid environments than is exhibited by currently used epoxy systems provided the extent of cure which occurs at these temperatures imparts acceptable chemical and physical properties to the resulting polyimide. Lower cure temperatures therefore would render the polyimide composition a preferred alternative to epoxy materials which cure at temperatures of about 350.degree. F., in those end-use applications involving exposure to hot humid environments at temperatures up to about 180.degree. F. and where stability upon sporadic increases in temperature up to about 350.degree. F. must be exhibited.
It would be a further advantage to reduce the cure time currently employed for preparing polyimides at the standard second stage curing temperatures used in preparing polyimides (eg. about 600.degree. F.). Standard curing temperatures would be employed if one wanted to take maximum advantage of the thermooxidative resistant properties obtainable from polyimides at temperatures of about 500.degree. to 550.degree. F. In such instances, the same materials currently used in curing polyimides would continue to be used.
While many catalysts are known to facilitate the crosslinking reaction of unsaturated species, the selection of an appropriate catalyst to be used in a process conducted generally in accordance with U.S. Pat. No. 3,745,149 for preparing crosslinked polyimides is complicated by the requirement that such a catalyst must be added to the mixture of all the monomers prior to their polymerization to form even the prepolymer. This requirement is necessitated by the fact that once the prepolymer is formed it is extremely difficult to uniformly disperse a catalyst therein particularly in a solventless system. Thus, a suitable catalyst must not only be soluble in the mixture of monomers it must also be inactive during much of the prepolymer formation in the sense that it does not cause the double bonds present on the monomers to react, and non-volatile at the relatively elevated temperatures employed over the entire course of the two stage polymerization process. In addition, a suitable catalyst must be capable of being activated only upon completion of the prepolymer formation. The extent of the crosslinking reaction which is induced by the catalyst at the second stage cure temperature is preferably sufficient to impart enough strength to a polyimide prepreg composite that it can be used directly with no further curing or alternatively can eventually be subjected to a free standing post-cure. The ability to undergo a free standing post-cure is desired since currently used prepregging epoxy resins can meet this requirement and a viable commercial substitute for epoxy resins in a prepregging system should also be amenable to this type of procedure. Obviously, where no post-cure is necessary an even greater advantage over epoxy resins is exhibited.
To date, a catalyst which meets all of the above requirements has not yet been identified.
While U.S. Pat. Nos. 3,575,924 and 3,671,490 disclose broadly that peroxide catalysts can be employed in the process described therein, the requirements of this process are different from those of the present invention used to prepare the polyimide, since the peroxide catalysts of the above described patents are added to a polyamide acid (e.g. the reaction product of an aryl dianhydride and an arylamine) and not to a mixture of monomer reactants of the type described herein. In the past, aromatic amines, such as phenyl-.beta.-naphthylamine which is similar to the arylamine monomers employed in the present invention have been used as an antioxidant to scavenge free radicals in a variety of environments. It would therefore be expected that peroxide catalysts would be rendered ineffective by reaction with the aromatic amine monomers.
Cyclic peroxides such as illustrated in R. Pastorino et al. "Cross-linking HDPE With Cyclic Peroxyketals", Modern Plastics Vol. 55 pp. 86-88 (1978); and those illustrated in U.S. Pat. Nos. 3,117,166; 3,419,577; and British Patent Specification No. 1,329,859 are known in the art and many have been employed for catalyzing the reaction of a variety of unsaturated monomers as well as polyethylene, rubber and the like. None of these peroxides, however, are believed to have been employed in a polymerization process for perparing polyimides in accordance with the procedures described herein.
It is therefore an object of the present invention to provide a process for preparing polyimides which permits the use of cure temperatures which are sufficiently low that such polyimides may be processed and cured in accordance with techniques typically employed in connection with epoxy resins.
It is a further object of the present invention to provide a process for preparing polyimides which permits the use of shortened cure times as standard curing temperatures while still imparting good chemical and mechanical properties to the polyimides.
It is still a further object of the present invention to provide a composite structure which employs a polyimide resin matrix and which can be prepared using techniques typically employed in preparing carbon fiber-epoxy composites.
It is another object of the present invention to provide a polyimide forming composition capable of use in preparing composite structures.
These and other objects and features of the invention will become apparent from the claims and from the following description.